Concrete generally exhibits a low tensile strength and low fracture toughness. The ease with which cracks can nucleate and propagate in concrete under tension makes it imperative that, to the extent possible, concrete not be loaded in tension, and if unavoidable, some form of traditional reinforcement, such as rebar, be provided to take the tensile stresses. The latter is generally known as reinforced concrete.
An alternate method of reinforcement is by incorporating short, randomly distributed fibers in concrete such that the reinforcing fibers are distributed throughout the matrix and thus, a new composite material, known as micro- or fiber-reinforced concrete, is obtained. Fiber reinforced concrete has significantly improved energy absorption capability (often called toughness), impact resistance, and fatigue endurance, with greater resistance to cracking. It can also have better durability with an improved appearance.
Concrete has been reinforced with metal, steel and polymer fibers, in some cases strengthening the concrete and even making it blast resistant. Thread-like elements (fibers) of steel wire, having uniform corrugations along their entire length, have also been used for the reinforcement of concrete. Typically, steel fibers can be found in different forms: round (cut from wire), flat (sheared from steel sheets), and irregularly shaped from melt. Mechanical deformations such as crimping, adding hooks or paddles at their ends, or roughening their surface sometimes increases the bonding of the fibers to matrix.
Several problems have prevented fiber reinforced concrete from becoming widely adopted as a replacement for traditional steel reinforcement bar (rebar) and welded wire fabric. For example, straight fibers do not affect the concrete until large cracks develop, making them highly inefficient and not cost effective as compared to traditional reinforcement. Also, testing methods are not available for reliably characterizing tensile performance of fiber reinforced concrete. A physics based (not empirical) design approach, one that yields economically practical, safe and reliable designs while fully recognizing all limitations that gives engineers the confidence to specify micro rebar, has never been available. Finally, there is no method available to verify the distribution of the fibers in concrete to assure compliance with the specified design.
The combination of these shortfalls has limited fibers to use as shrinkage and temperature reinforcement in slabs on grade. While fibers have been implemented beyond slabs and have been used to replace tensile reinforcement in concrete, their acceptance has been limited by the problems described above.